The present invention relates to a power supply unit.
In portable electronic and electrical equipments such as personal computer, a secondary battery such as lithium ion battery and nickel hydrogen battery has been used as a power supply unit. However, the secondary battery cannot feed or supply electric power to the personal computer for longer than about four consecutive hours. Recently, a fuel cell capable of feeding electric power to the personal computer for 20 to 40 consecutive hours has been getting a lot of attention.
A circulating type unit is a typical unit of a fuel cell using methanol as fuel. FIG. 9 is a block diagram showing the configuration of a prior art circulating type fuel cell. FIG. 9 shows a return pump 111, a dilution tank 112, a methanol pump 113, a methanol tank 114, a fuel cell 116, a fuel cell control part 117 and a gas-liquid separator 901. The fuel cell 116 has a stack 122, a fuel pump 123 and an air pump 124.
The methanol tank 114 stores methanol (CH3OH) of several % to 100% component therein.
The methanol pump 113 feeds methanol from the methanol tank 114 into the dilution tank 112 according to an instruction of the fuel cell control part 117. The dilution tank 112 dilutes methanol of several % to 100% to 5% wt. The fuel pump 123 feeds the diluted methanol from the dilution tank 112 into the stack 122 according to an instruction of the fuel cell control part 117. The air pump 124 feeds air into the stack 122 according to an instruction of the fuel cell control part 117.
In the stack 122, methanol is supplied to a fuel electrode (−) and air is supplied to an air electrode (+). At the fuel electrode (−), methanol reacts with water to form carbon dioxide, hydrogen ions and electrons in a region called as a three-phase interface where methanol and water as reactants, catalyst (electrode surface) and electrolyte are in contact with each other (CH3OH+H2O→CO2+6H++6e−). Hydrogen ions and electrons pass through a polymer membrane and an external load, respectively, and reach the air electrode (+). At the air electrode (+), oxygen in air reacts with hydrogen ion and electron derived from the catalyst (electrode surface) on the three-phase interface to form water (3/2O2+6H++6e−→3H2O).
The stack 122 discharges the used methanol of 3 to 5% wt, carbon dioxide and water from the fuel electrode of (−) side, and water and air from the air electrode of (+) side. The gas-liquid separator 901 separates carbon dioxide from methanol, carbon dioxide and water discharged from the stack 122 and discharge it. The return pump 111 feeds remaining methanol and water into the dilution tank 112. The methanol and water are reused to generate diluted methanol in the dilution tank 112.
Patent document 1 (Unexamined Patent Publication No. 2000-173636) discloses a prior art fuel cell apparatus to which electric power is supplied from a secondary battery at the time of sudden change in an external load. FIG. 10 is a block diagram showing the configuration of the fuel cell apparatus of the patent document 1. A fuel cell main unit 1001 uses hydrogen as fuel gas. When a sudden change in the load causes an output voltage of the fuel cell main unit 1001 to be lowered temporarily to become a predetermined voltage V3 or less, output to a charge control part 1006 is stopped at a circuit switch control part 1007 in response to a signal from a sensor part 1009. When the output value further lowers to become a predetermined voltage V4 or less, output to an auxiliary device 1002 is stopped at the circuit switch control part 1007 in response to a signal from a sensor part 1009, and at the same time, the output power to the auxiliary device 1002 is switched to the output power of a secondary battery 1005. The fuel cell apparatus of the patent document 1 is controlled so as to supply power stably to the external load based on the output voltage of the fuel cell main unit 1001.
Patent document 2 (U.S. Pat. No. 2,775,890) discloses a prior art control device of fuel cell power generation system that maintains a storage battery 1106 in a state of a target charged capacity (for example 80 to 90%). FIG. 11 is a block diagram showing the configuration of the control device of fuel cell power generation system of the patent document 2. A discharge power amount computing device 1115 fetches an output of a storage battery current detector 1111 and calculates discharged charge amount of a storage battery 1106. A controller 1116 determines power to be generated from the fuel cell so as to supply power corresponding to the discharged charge to the storage battery 1106 within a period at every specific cycle, and outputs a control instruction to an auxiliary controller 1110 and a DC-DC converter 1104. The control device of fuel cell power generation system of the patent document 2 can supply power to the load stably while maintaining the storage battery 1106 in a state of a target charged capacity, based on the discharged charge of the storage battery 1106.
Patent document 3 (Unexamined Patent Publication No. 2000-12059) discloses a prior art fuel cell system and fuel cell control method that make a fuel cell operate at an operating point with the highest energy conversion efficiency. FIG. 12 is a block diagram showing the configuration of the fuel cell system of the patent document 3. A reformer 1228 generates hydrogen-rich gas (reformed gas) containing hydrogen therein by steam reforming reaction of methanol injected as fuel 1224 and water. The fuel cell 1236 generates electric power using the hydrogen-rich gas as fuel gas. A control part 1220 calculates an operating point with the highest energy conversion efficiency in the fuel cell 1236 based on the gas flow rate and makes the fuel cell 1236 operate at the operating point.
The prior art circulating type fuel cell has the deficiency of discharging a considerable amount of methanol along with carbon dioxide because of the difficulty in separating only carbon dioxide from the used fuel. For this reason, less than 10% of the amount of supplied methanol becomes available power, resulting in a low fuel utilization rate (details to be described later).
In the fuel cell, some delay occurs until the output power of the fuel cell is increased or decreased by increasing or decreasing the amount of fuel supplied to the fuel cell. In the fuel cell apparatus of the patent document 1 for example, the fuel cell is required to change the output power immediately in response to a change in the load. In the fuel cell apparatus of the patent document 1, it is necessary to continue to supply fuel of the amount much greater than a required amount to the fuel cell so as to cope with a sudden change in the load. This results in lowering of fuel utilization rate of the fuel cell. The fuel cell using methanol as fuel has the problem of discharging a large amount of methanol.
In the control device of fuel cell generation system of the patent document 2, since the generated power of the fuel cell is changed with a short cycle, it is difficult to control it, thereby to complicate the configuration of the fuel cell. As the fuel cell system and fuel cell control method of the patent document 3 require a reformer, the problem is that the system is costly and large-sized. In the patent document 3, the fuel cell 1236 is made to operate at the point with the highest energy conversion efficiency (=generating efficiency×gas utilization rate). When the fuel cell system and fuel cell control method of the patent document 3 is applied to the non-circulating type DMFC (Direct Methanol Fuel Cell), because sufficient fuel has to be supplied, a large amount of unused methanol is discharged from the fuel cell. As a result, a problem arises with the method of cleansing the discharged methanol.
A non-circulating type fuel cell is a fuel cell in which fuel is not circulated and used fuel is discharged. In such fuel cell, methanol supplied from an entrance of the fuel cell is consumed gradually and discharged from an exit of the fuel cell. However, when the supplied methanol lacks with respect to output current, the output voltage of the fuel cell is lowered rapidly. To output power stably and cope with a sudden change in the load, a large amount of unused methanol is discharged from the conventional non-circulating type fuel cell. Nevertheless, as methanol has toxicity, it cannot be discharged as it is. Since a certain amount of unused fuel is discharged, the non-circulating type fuel cell has been deemed to be unsuitable for the fuel cell using toxic methanol as fuel.
The present invention is devised in consideration with the above-mentioned problems and intends to provide a clean power supply unit.
The present invention intends to provide a power supply unit having a good fuel utilization rate.
The present invention intends to provide a power supply unit capable of supplying electric power stably in response to a power change in the load without largely changing the power generated by the fuel cell.
The present invention intends to provide a power supply unit of simple configuration.